Secondary recovery of oil from underground reservoirs



mms W Nov. 1, 1938.

INVENTOR Gear* e 6. Bs

BY xm ATTORNEY 8,000 lbs/sq. in., and preferably between about- 2,500lbs/sq. in, and about 5,000 lbs/sq. in.

The gas is allowed to remain in contact with the oil in the depletedreservoir at this pressure for a period of at least several days andpreferably several months to permit approximate equilibrium to takeplace within the very Vilne pores and capillaries of the reservoir.

When this period has been allowed in order to achieve approximateequilibrium, gas is produced from the formation. This gas may beproduced from the same wells through which it is introduced and thisconstitutes an additional advantage over, and distinction from, gasdrive and water drive methods which necessarily use separate wellsI forthe driving fluidv and the oil produced.

Thegas is produced at as near reservoir pressure as practicable. In anyevent the pressure in the formation surrounding the well or wells fromwhich the gas is produced is not allowed to drop below 1,500lbs./sq'.`in. and preferably not belou` about 2,500 lbs/sq. in. It isalso desirable that the pressure within the well itself be maintainedabove Vthese pressures but this is not always essential sincecondensation within the well results in entrainment and the liquid thusseparating out is not necessarily lost as it would be if thecondensationoccurred within the underground formation.

Due to the retrograde condensation phenomenon, or what is sometimesreferred to as retrogradekrvaporizatiomthe gas produced under highpressure contains normally liquid hydrocarbons from the undergroundfomiation' whichl were dissolved in the gas by virtue of the highpressure. These hydrocarbons can be separatedv from the gas by merelylowering the pressure to some value yin the neighborhood of the criticalretrograde condensationpressure, for instance a pressure of from about400 lbs/sq. in. to about 1 ,200 lbs/sq. in., and preferably from about600lbs./sq. in. to about 1,000 lbs/sq. in., and then separating thevapor and liquid phases. If the pressure is lowered below the criticalretrograde condensation pressure, the liquid phase commences to reenterthe vaporvphase which is, of course, undesirable since the objective isto separate the maximum amount ofliquid hydrocarbons.

The pressure at which the liquid and vapor phases are separated shouldbe at least about 500 lbs/sq. in., andpreferably at least about 1,000lbs/sq. in. below the minimum pressure within the reservoir. ispreviously indicated, at least a part of this pressure Ydrop 'can occurwithin the well but the bulk of it preferably occurs above ground. Y

While it is'possible, as above mentioned, to use the same well for theintroduction and withdrawal of gas in accordance with my invention, thisnecessitates'discontinuous operation, and it is greatly preferable tousedinerent wells. Since in practicing my invention the fluid leaving theformation is homogeneous and at least substantially free from anyseparate liquid phase', the point of gas injection can be either higheror lower l on the reservoir than the point of withdrawal.

'Villar reference .A simplied and highly diagrammaticl illustration ofone type of equipmentl for practicing my invention is show in theaccompanying drawing whichforms fa part of `this specification. and isto be read in connection therewith. My linvention will be furtherdescribed with particto thedrawlng. y -V i.v ilintroducedatbighpressureinto well A", passes through the partially depleted oil reservoir I0,which is isolated by impervious strata II and I2, and by brine barrierI3, passes into well B" at high pressure, is withdrawn from well Bthrough line I4, and has its pressure reduced to form a liquid phase byvirtue of the retrograde condensation effect. This liquid phase isremoved by means of separator I5, the gas is recompressed and thenre-introduced into well A.

The gas introduced into Well A can be composed in whole or in part ofre-cycle gas withdrawn through well ,B as we have just seen, or all orpart of it can come from a separate source, for instance, valved line28.' In anyY event, an extraneous gas is usually necessary for makeuppurposes, since there is inevitably some slight loss. The gas introducedis constituted wholly or at least predominantly of one or morehydrocarbons having one or two carbon atoms per molecule. Usuallymethane is the main constituent, but ethane can be used. Natural gas, orother mixtures of methane and ethane, with or without minor quantitiesof` heavier hydrocarbons, are the mcst suitable and available materials'for use in practicing my invention.

This gas can be introduced at any pressure above about 1,500-2,500lbs/sq. in., butsubstantially higher pressures are desirable to providepressure drops since these figures represent the minimum pressures whichshoul be built up within the reservoir.

As previously indicated the'gas picks up heavier hydrocarbons (largelypropane to heptane) from the otherwise unproducible oilillms existingwithin the partially depleted oil reservoir I0,

and then issues .throughwell B and line Il.v

ably from Vabout 600 lbs/sq. in. to'about 1,000

lbs./sq. in. I This pressure reduction is, of course, accompanied by atemperature reduction andthis temperature drop causes further amounts ofliquid phase to separate. In fact, further cooling by external means isdesirable to gain still further liquid phase precipitation and stillhigher recoveries of hydrocarbons from vpropane to heptane and heavier.The temperature at which the liquid phase is separated and removed maysuitably be from about 25 F. to about 150 F. Temperature adjustment 'isobtained by means of cooler I1, which may precede or follow pressurereduction valve I6, but preferably precedes it since cooling medium ofhigher temperature can then be used.

As a matter of fact, heavier hydrocarbons picked up by virtue of theretrograde condensation eil'ect can be precipitated by cooling, withoutpressure reduction, and while this does not give as high recoveries asare obtained with pressure drops, recompression costs are reduced whichcompensates for the low recoveries to some extent. v

Following pressure and/or temperature adjustment, the fluids enterseparator I5, which may be of any conventional type. From this separatorthe liquid phase, separated by virtue of the retrograde condensationphenomenon is removed through valve Il, which is responsive to liquidlevel control I 8, and then passes'through line 20 tion effect.

system of stage separation for removal of methane' and ethane which canbe recycled. n

Gas, stripped of a major portion of its components heavier than ethane,passes through back pressure valve 2|. It can then be sent through valve22 and line 23 to a further recovery system ofrconventional type,wherein additional quan@ tities of hydrocarbons heavier than ethane canbe recovered by methods such-as refrigeration, char?,l coal adsorption,or oil absorption. After passing through this supplemental recoverysystemthe dry gas can be re-introduced into well A. i

However, the retrograde condensation phenomenon, properlyutilized,`sufiices to recover the greatbulk of the valuable heavierhydrocarbons and the gas from separator I5 can be, and preferably is,recycled directly to Well A through valve 2,4 and compressors 25. Itwill be seen that by opening valve 25 and closing by-pass valves 21, thecompressors can be used in series, while by closing valve 26 and openingthe by-pass valves, they can be used inparallel.r o

While I have described my'invention in connection with certain specicembodiments thereof, it is to be understood that these are by way ofillustration and not by way of limitation, and the scope of my inventionisdened only bythe appended claims which should bcv construed as broadlyas the prior art will permit.

I claim: n l

1. A method of recovering oil components vfrom a partially depleted,underground oil reservoir which comprises introducing natural gas ,intosaid reservoir at a pressure of' from about 1,500 lbs ./sq.,in. to about8,000 lbs/sq. in`.,.maintaining said gas in contact with the oil in`said reservoir under pressures of this same order of magnitude for asubstantial period of time and causing said gas, substantially free fromany separate liquid phase, to pass from said reservoir into a well whilemaintained at a pressure in Aexcess of labout 1,500 lbs/sq. in., saidgas carrying with itdissolved liquidhydrocarbons picked up from saidreservoir by virtue of the retrograde condensa- 2. A method ofrecoveringoil components from a partially depleted underground oilreservoir which comprises introducing naturalgas into saidreservoirthrough a rst well at a pressure from about 1,500 lbs./sq, in. to about8,000 lbs/sq. in., passing said gas through said reservoir whilemaintaining its l pressure above about 1,500 lbs/sq. in. and passingsaid gas together with heavier hydrocarbons picked up from saidreservoir by virtue of the retrograde condensation effect into a secondwell while maintained at a pressure in excess of about 1,500 lbs/sq. in.

3. A method of recovering oil components from a partially depletedunderground oil reservoir which comprises introducing natural gas intosaid reservoir at a. pressure of from about 1,500 lbs./ sq. in. to about8,000 lbs/sq. in., maintaining said gas in contact with the oil in saidreservoir under pressures of this same order of magnitude for asubstantial period of time, causing said gas to pass from said reservoirinto a well While maintained at a pressure in excess of about 1,500lbs/sq. in., lowering the pressure of said lastmentioned gas by at leastabout 500 lbs/sq. in. i

to a value Within the range from about 400 lbs/sq. in. to about 1,200lbs/sq. in. to ,separate liquid hydrocarbons by virtue of the retrogradecondensation eifect and removing the separated liquid hydrocarbons fromthe remaining gas.

4. A method of recovering oil components from a partiallyAdepletedunderground oil reservoir whichcomprises introducing gas rich inmethane into the said reservoir at Aa pressure ofvv from about-2,500.lbs/sq. in. to about 5,000vlbs./sq. in; maintaining said.gas incontactwith the oil in said reservoirvunder pressure of this same orderof magnitude .for a substantial period of` time, causing said gasto passfrom said reservoir into a well while maintained at afpressure'in excessof about 2,500 `lbs/sq. in., lowering the pressure on `said.last-mentionedgas by fat-least about 1,000 lbs./sq.in. toa value withinthe range from about 600 lbs/sq. in., toabout 1,000 lbs/sq. in. toseparate liquid hydrocarbons byvirtue of the retrograde condensationeffect'and removing the separated liquid hydrocarbons from the remaininggas. o f

5. A method of recovering oil components from a partially depletedunderground oilrreservoir which comprises introducing gas composed atleast predominantly of at least Onehydrocarbon having not more than twocarbon atoms per molecule into said reservoir througha rst well at Aapressure of from about 1,500 lbs/sq. in. to about 8,000 lbs/sq. in.,passing said gas through said reservoir while v maintaining its pressureabove about 1,500 "lbs/sq. in., passing said gas together with dissolvedheavier hydrocarbons picked up fromfsaid reservoir into a, second wellwhile maintainedat a pressure in excess of about l1,500 lbs'j/s'q in.,issuing said gas from said second well lowering the' pressure on saidgas by at least about 500 lbs/sq.` in. to a value' within the range fromabout Y400 lbs/sq. in. to about 1,200 lbs/sq. in'. to separate liquidhydrocarbons by virtue of the retrograde condensation effect andremoving the separated liquid hydrocarbons from the remaining gas. n

S. A method according to claim 5 in which the gas issuing from saidsecond "well is cooled by 'external means .prior to the removal ofseparated liquid hydrocarbons, thereby increasing the amount of liquidhydrocarbons recovered. e 7. A method of recovering oil components froma partially depleted underground oil reservoir which comprisesintroducing gas of the natural gas type into said reservoir through afirst well ata pressureof from aboutr1,500 1bs./sq. in. to

about 8,000 lbs./sq.`in., passing said-gas throughl said reservoir whilemaintaining its pressure aboveabout 1,500.lbs./sq.-in., passing said gastogether with dissolved heaviery 'hydrocarbons picked up from saidreservoir into a second well while maintained at a pressure in excess ofabout 1,500 lbs/sq. in., issuing said gas from said second well,lowering the pressure on said gas by at least about 500 lbs/sq. in. to avalue within the range from about 600 lbs/sq. in. to about 1,000 lbs/sq.in. to separate liquid hydrocarbons by virtue of the retrogradecondensation effect and removing the separated hydrocarbons from theremaining gas.

8. A method according to claim 7 in which said second well is higher onsaid reservoir than said first well.

9. A method of recovering oil components from a partially depletedunderground oil reservoir whichl comprises introducing natural gas intosaid reservoir through a first well at a pressure v from about 1,500lbs/sq. in. to about 8,000 lbs./ sq. in., passing said gas through saidreservoir while maintaining its pressure above about 1,500 lbs/sq. in.,passing said gas together with heavier hydrocarbons picked up from saidreservoir by virtue of the retrograde condensation eiect into asecondwell while maintained at a pressure in` excess of about 1,500 lbs/sq.in., recovering at leastl a substantial ypart of said heavierVhydrocarbons, recompressing at least a substantial part of theremaining gas and recycling said recompressedv gas. to said reservoir.'y

10. A method of recovering oil components y from a partially depletedunderground oil reserl the pressure on said gas to a value within thevoir which comprises introducingy natural gas into saidreservoirthroughaiirst well at a pressure ofy from about 1,500- lbs/sq. in.totabout 8,000 iba/sq. in.,` passing said gas throughV said reservoirwhile maintaining its pressure above Y about 1,500 lbs/sq. in., passingsaid gas together withy heavier hydrocarbons picked up from 'saidreservoir into'a'secondwell while `maintained at a pressure in excessoiabout 1,500 lbs/sq. in., issuing said gas from said secondwell, loweringrange from about 400 lbs/sq. in. to about 1,200 lbs/sq. in. to separateliquid hydrocarbons by virtue of the retrograde condensation reiIect,removing the separated liquid hydrocarbons from the remaining gas,recompressing atleast a subjstantial part of said remaining gas to apressure in excess oi the reservoir pressure, and recycling saidrecompressed gas to said reservoir.

11. A `method of e recovering; oil components `irom a partially depletedunderground oil reservoir vwhich comprises introducing natural gas intosaid reservoir through a rst well at a presof from about 2,500 lbsl/sq.in.` to about 5,000 lbs/sq. in., passing said gas' through saidreservoir while maintaining its :pressure above about`2,500 lbs'./sq.in., passing said, gas'together with dissolved heavier hydrocarbonspicked up from said reservoir into a second wellwhile maintained at a.pressure in excess of about 2,500 lbs/sq. in., issuing said gas fromsaid second Well, lowering the pressure on said gas to a value withinthe range from about 600 lbs./sq. in. to about 1,000 lbs/sq. in. toseparateliquid hydrocarbons by virtue of lthe retrogradecondensationeiect, removing the separated liquid.

hydrocarbons from the remaining gas, recompressing at least asubstantial part of said; re-

maining gas to a pressure of from about 2,500 lbs/sq; in. to about 5,000lbs/sq. in., and recycling at least asubstantial part oi saidrecompressed gas to 'said reservoir. Y

12. A method of recovering oil components from a partially depletedunderground oil reserlvoir which comprises introducing gas composed atleastpredominantly o! at least one hydro? l carbon having less thanthree carbonv atoms'per molecule into said reservoir through a ilrstwell at a pressure of from about 2,500 lbs/sq. in. to

about 5,000 lbs/sq. in.,` passing saidlg'as throughtA pressing at leasta substantial part of said re" 'maining gas to a pressureA of from about2,500

lbsQ/sq. in. to about 5,000 lbs/sq. in., and re-S` cycling at least asubstantial partofsaid recom# vpressed gas to said reservoir. .v

13,. A method of ,recovering oil components from a partially depletedunderground oil reservoir which comprises introducing gas composed atleast predominantly oi.' at least one hydro#- carbon having less thanthree carbon vatoms per molecule into said` reservoir through a firstwell at a pressure of from about 1,500 1bs./sq.in. to about 8,000lbs/sq. in., passingv said gas through saidv reservoir while maintainingits pressure above about 1,500:1bs./sq. in., passing said gas togetherwith dissolved heavier hydrocarbons vpicked upirom said reservoir into asecond well while maintained at a pressure inexcessof about1,500'1bs./sq. in., `issuing said gas from r`said seccond'well,adjusting the pressure andtemperature of said gas to obtain av maximumamount'of Vliquid phaseeseparation Y'byv virtue of the' retrogradecondensation eiect and removing the separated liquid phase. s

14,. A method according to claim 13 in which .K

said adjusted pressure'is within the range from about 400 lbs/sq. in.'tov about v1,200 lbs/sq. in. and said adjusted temperature is within therange from about 25 F. to about 150'F.

15.` A method according to claim 13 in which said adjusted pressure iswithin the range from about 600 lbs/sq. in. to about 1,000 lbs/sq. in.and said adjusted temperature is within: the range from about 25 F. toabout 150 F.

GEORGE s. BAYs.

